1280 HYDROQUINONE INDOLE. 



of very dilute solution of ferric chloride, avoiding all excess of 

 the reagent. If the green solution is now acidulated with 

 tartaric acid, it turns violet on the subsequent addition of a 

 little ammonia and purplish-red on the addition of excess. The 

 green colour may be restored by excess of acetic acid. It may 

 be distinguished from hydroquinone by yielding a precipitate 

 with normal acetate of lead which is soluble in acetic acid, 

 whereas the latter substance does not. 



But little is known as to the source of this substance in 

 urine apart from its probable formation from the phenol pro- 

 duced by putrefactive changes in the alimentary canal. In 

 herbivora there is some evidence that it is derived from certain 

 aromatic constituents of their food. 



9. Hydroquinone. C 6 H 4 (OH) 2 . Para-dihydroxybenzene. 



Has not yet been described as occurring normally in urine, 

 but only as the result of the ingestion of phenol. It exists in 

 urine as an ethereal compound with sulphuric acid and is 

 largely the cause of the dark colour which this excretion as- 

 sumes after the absorption of phenol on exposure to the air. 

 It resembles pyrocatechin in effecting the reduction of metallic 

 salts, but differs from it in being nearly insoluble in cold ben- 

 zene and in not yielding any precipitate with normal lead 

 acetate. This latter property suffices for its separation from 

 pyrocatechin. It is readily converted by oxidation into qui- 

 none, C 6 H 4 O 2 , whose characteristic odour affords a further 

 means of identification, and when heated in an open test-tube it 

 yields a blue sublimate. 



The third known isomeric dioxybenzene viz. meta-dihy- 

 droxybenzene or resorcin has not yet been found in the -animal 

 body or in urine. 



THE INDIGO SERIES. 



1. Indole. C 8 H 7 N. 



Indole occurs characteristically in the fteces, to which with 

 skatole it imparts their peculiarly unpleasant odour. Its 

 presence here is due to its formation during the putrefactive 

 decomposition of proteids which usually occurs to a greater or 

 less extent in the alimentary canal, part of the indole leaving 

 the body in the urine as a potassium salt of indoxylsulphuric 

 acid (see below), the remainder being excreted with the faeces. 

 It may readily be obtained, contaminated by varying quantities 

 of phenol and skatole (see below), by acidulating and distilling 

 the products of a not too prolonged alkaline putrefactive pan- 



